The present invention relates to a new and improved tube, rod or the like produced from silica glass or other glass-like materials and a method for making the same. Particularly, this invention relates to a method for the production of elongated members from a silica melt. The present invention is particularly directed to the fusing of a silica deposition tube in a deuterium (D2) gas atmosphere for decreased fiber attenuation. Alternatively, the deposition tube can be formed from a silica sand pretreated in a deuterium (D2) gas atmosphere prior to fusing of the deposition tube.
Various types of elongated members have been formed continuously by melting of silica crystal or sand in an electrically heated furnace whereby the desired shape is drawn from the furnace through a suitable orifice or die in the bottom of the furnace as the raw material is melted. One apparatus for continuous production of fused silica glass tubing, for example, is a tungsten-lined molybdenum crucible supported vertically and having a suitable orifice or die in the bottom to draw cane, rods, or tubing. The crucible is surrounded by an arrangement of tungsten heating elements or rods which heat the crucible. The crucible, together with its heating unit, is encased in a refractory chamber supported by a water-cooled metal jacket. The crucible is heated in a reducing atmosphere of nitrogen and hydrogen.
An alternative apparatus provides fused silica glass tubing by feeding natural silica crystal into a refractory metal crucible heated by electrical resistance under a particular gas atmosphere to reduce the bubble content. The bubbles formed by gas entrapment between crystals and the molten viscous mass of fused silica glass do not readily escape from the molten glass and, hence, remain as bubbles or lines in the product drawn from the fused silica melt. By substituting a melting atmosphere gas which readily diffuses through the molten material (such as pure helium, pure hydrogen or mixtures of these gases) the gas pressure in the bubbles are reduced and thereby the bubble size is reduced. This process uses a mixture of 80% helium and 20% hydrogen by volume.
In a further alternative method, a product is obtained by continuously feeding a raw material of essentially pure silicon dioxide in particulate form into the top section of an induction-heated crucible, fusing the raw material continuously in an upper-induction heat zone of the crucible in an atmosphere of hydrogen and helium while maintaining a fusion temperature not below approximately 2050xc2x0 C. The fused material in the lower zone of the crucible is heated by separate induction heating means to produce independent regulation of the temperature in the fused material. The fused material is continuously drawn from the lower zone of the crucible through forming means in the presence of an atmosphere of hydrogen containing a non-oxidizing carrier gas.
Deposition tubes manufactured in accordance with the above-referenced processes necessarily contain hydrogen, since hydrogen is used to protect refractory metal-made furnace parts from oxidation at high temperatures. The hydrogen from the process dissolves into the silica glass. Moreover, the tube inherits a considerable amount of OH from the silica sand itself. As a result, a large quantity of hydrogen is present in the tube drawn from the furnace. The presence of hydrogen in a deposition tube, however, is problematic in that it deteriorates the optical attenuation of the tube.
More particularly, the fused silica glass formed in the above-described processes can be used in the production of a fiber optic preform by the well-known MCVD process wherein the silica glass is in the form of a tube surrounding a core which is built up by vapor deposition of successive layers of suitably doped silica on the inner wall surface of the tube. The tube, after drawing, becomes the outer sheath or cladding of the fiber.
During the MCVD process, any hydrogen present in the tube diffuses towards the deposited optical core and forms various H-bearing species. These species give rise to absorption peaks that adversely affect light transmission at the 1.3 and 1.55 xcexcm communication windows. The strong OH absorption at 1.38 xcexcm, in addition, prohibits the use of the 1.2-1.6 xcexcm wavelength. Elimination of this peak therefore would significantly expand the wavelength range for lightwave transmission. Moreover, as a small fraction of the light is transmitted in the cladding, the presence of hydrogen in the deposition tube itself also deteriorates the attenuation.
One approach for driving out the dissolved hydrogen from the tube entails a heat treatment in a vacuum or other gas environment. However, due to the slow rate constant of the de-hydrogen process at low hydrogen concentration, there is always residual hydrogen left in the tubexe2x80x94typically up to 10xe2x88x926 mol/cm3xe2x80x94even after a lengthy heat treatment.
Another approach for reducing the hydrogen content comprises shifting the relevant vibration modes of hydrogen to longer wavelengths to reduce the absorption due to hydrogen in the wavelength region of interest. Such a shift is known to occur by substitution of a heavier atom for hydrogen. Substituting deuterium for hydrogen has the desired effect, since deuterium has approximately double the mass of hydrogen. The deuterium/hydrogen isotope (D/H) exchange results in the appearance of OD absorption lines in the relevant wavelength regime. However, these lines are due to higher overtones, and thus weaker by typically 1-2 orders of magnitude.
A D/H isotope exchange in the tube subsequent to its formation is problematic from a manufacturing standpoint in that it will not effect an isotope exchange of 100%. Moreover, the exchange must be done at a high temperature and high pressure and, in that regard, is difficult to control, particularly when a flammable gas is used. It would therefore be desirable to have available a method for fusing of a silica article in a substantially hydrogen-free gas atmosphere for improved attenuation performance.
In an exemplary embodiment of the invention, a method for forming an elongated fused silica article is provided. The method generally comprises feeding a silica or quartz (SiO2) material into a furnace. The SiO2 material is fused in a melting zone of the furnace under a gas atmosphere including molecular deuterium (D2) gas. The article is then drawn from the furnace.
In an alternate exemplary embodiment of the invention, a method for forming an elongated fused silica article is provided wherein quartz (SiO2) material is pretreated with a molecular deuterium gas to undergo D/H exchange in the quartz (SiO2) material prior to fusing of the SiO2 material into the drawn article. This pretreatment step is followed by feeding the pretreated quartz (SiO2) material into a furnace and fusing of the material in a melting zone of the furnace under a gas atmosphere including a deuterium gas or a substantially hydrogen-free gas atmosphere. The article is then drawn from the furnace.
In still another exemplary embodiment of the present invention, an elongated fused quartz article is formed by fusing of a generally quartz (SiO2) material in a gas atmosphere comprising molecular D2 gas.
A principal advantage of the invention is provided by a new and improved silica tube or rod that, typically contains hydrogen-bearing species less than {fraction (1/100)} as compared with those fused in the presence of hydrogen, thereby improving the attenuation of the tube. Because the tube or rod is formed in a molecular D2 gas atmosphere, hydrogen is substantially absent in the tube as formed, thus negating the necessity of a D/H isotope exchange.
It should be noted that the terms xe2x80x9cquartzxe2x80x9d and xe2x80x9csilicaxe2x80x9d are used interchangeably throughout this application, both being directed generally to the compound SiO2. Nonetheless, the present invention encompasses the use of any raw material introduced to the melting furnace, including but not limited to natural silica/quartz and synthetic silica.
It should further be noted that the term xe2x80x9cdeposition tubexe2x80x9d is used throughout this application. However, one of ordinary skill in the art would recognize that the inventive tube is also known as a substrate tube or a cladding tube.